The present invention relates to a process for the purification of bisphenols.
Bisphenols are valuable compounds useful in the preparation of various polymers, such as epoxy resins or polycarbonates. High quality epoxy resins, and particularly polycarbonates, require especially pure bisphenols for use in their preparation. Bisphenols are prepared according to various known processes by the condensation reaction of an aldehyde or a ketone and a stoichiometric excess of a phenol in the presence of a catalyst. These known processes produce bisphenols and certain impurities including isomers, analogs and homologs, such as 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (hereafter referred to as o,p-bisphenol isomer), 2,2,4-trimethyl-4-(4-hydroxyphenyl)chroman, trisphenol, polyphenol and unfavorably colored substances.
Numerous processes exist for the purification of bisphenols in general and particularly for the purification of 2,2-bis (4-hydroxyphenyl)-propane (hereafter referred to as bisphenol A or the p,p'-isomer).
U.S. Pat. No. 3,919,330 discloses a purification process wherein crude bisphenol A crystals are dissolved in an organic solvent. Water is then added, the solution is cooled, bisphenol A crystallizes and the crystals are then separated by filtration or centrifugation. U.S. Pat. No. 4,354,046 relates to a process for purifying bisphenol A wherein crude bisphenol A is mixed with toluene and a critical amount of water. The critical amount of water is 2-9% by weight based on the amount of crude bisphenol A. This mixture is heated to 80.degree.-100.degree. C. to form a single liquid phase. The liquid phase is then cooled whereby bisphenol A crystallizes. However, some of the organic solvents which have been suggested for the purification of bisphenols are inflammable or toxic.
A method which uses water as the crystallization medium for bisphenol A is disclosed in U.S. Pat. No. 3,326,986. According to this patent, crude bisphenol A is mixed with water at a ratio of 0.5 to 2.0 parts, preferably 1.0 part of water per part by weight of crude bisphenol A. The mixture is heated to a temperature of about 100.degree. C. resulting in an aqueous phase and a liquid organic phase. The mixture is cooled slowly to crystallize bisphenol A. Any isomeric diphenols or other organic impurities remaining in the crystals are removed by washing with a chlorinated solvent, such as chloroform, methylene chloride, ethylene dichloride, propylene dichloride or chlorobenzene. However, the necessity to wash the crystalline bisphenol A with a chlorinated solvent for obtaining a sufficiently pure product is undesirable.
In order to overcome the disadvantages of the method taught in U.S. Pat. No. 3,326,986, U.S. Pat. No. 4,461,915 suggests mixing water-crystallized bisphenol A in the presence of water with a water-immiscible organic solvent, such as toluene, agitating the mixture and forming three phases in the agitated mixture. The phase containing mainly the organic solvent is removed and purified bisphenol A is recovered from the remaining two phases. However, the usage of large amounts of an organic solvent, such as toluene for purification purposes, is undesirable.
U.S. Pat. No. 4,507,509 suggests a method of purifying bisphenol A by washing solid crude bisphenol A with an aqueous alkaline solution. The concentration of the base ranges from 0.1 to 25 weight percent of the crude bisphenol A. However, the use of an alkaline solution for washing purposes may affect the quality of the purified bisphenol A.
U.S. Pat. No. 4,740,635 discloses a process for crystallizing bisphenol A wherein water is added to a mixture of a phenol-free mixture of bisphenol A, 0.5 to 15 weight percent diphenol isomers and impurities. The ratio of water to the crude bisphenol A mixture is between 1:1 and 3:1, preferably between 1.5:1 and 2.5:1. Water and the crude bisphenol A mixture are heated to a temperature of 95.degree.-105.degree. C. at ambient pressure to melt all the solid material. Then it is adiabatically cooled, while stirring, by reducing the pressure. The temperature is brought down to below 90.degree. C., preferably to a temperature of from 55.degree. to 75.degree. C. The crystallized bisphenol A may be washed to further increase its purity. Unfortunately, the maximum purity of the crystallized bisphenol A, even after several washing operations, does not exceed 99.2%. U.S. Pat. No. 4,927,973 discloses a process for the continuous purification of crude bisphenol wherein, in a first step, a single liquid phase is formed which comprises a hot mixture of crude bisphenol and water in a weight ratio of 75-85% of crude bisphenol to 15-25% of water. In a second step, this mixture is continuously fed together with a stream of warm water to a first crystallization zone at a temperature sufficient to maintain a bisphenol-rich liquid phase, a water-rich phase and a crystalline phase comprising 70-95% of the total bisphenol in the mixture. When the mixture in the second step comprises 15-20 weight percent of bisphenol and 80-85 weight-percent of water, the temperature in the second step is 99.degree.-101.degree. C. The three-phase mixture is passed to a second crystallization zone operated at 85.degree.-97.degree. C. until 90-99% of the total bisphenol is crystallized. Bisphenol of very high purity is recovered, however, in the described process the bisphenol used as a starting material already has a high purity. The total amount of impurities less phenol according to the examples in the U.S. patent is only about 330 ppm, however, the total amount of impurities in the bisphenol prior to the purification process is only 1700 ppm. Single crystals and coarse agglomerates are said to be obtained in the process. However, the presence of coarse agglomerates is undesirable because they generally include mother liquor with impurities. Furthermore, the presence of agglomerates reduces the washing efficiency when the recovered crystals are washed.
U.S. Pat. No. 4,533,764 discloses a process for removing occluded organic solvent from bisphenol crystals. The crystals are placed in water which is maintained at a temperature of 100.degree. C. or above to produce a molten water-bisphenol phase. According to the Examples the weight ratio between water and bisphenol A is 3.4:1 or 4:1 respectively. The water-bisphenol phase separates from the excess water, and the solvent occluded by the bisphenol crystals can diffuse into the water phase from where it can be flash distilled. After the flash distillation the temperature of the bisphenol-water phase is reduced to allow the bisphenol to separate from the water and to crystallize. The described process is very useful for removing organic solvents which can be distilled off. However, the described process is less useful for removing impurities which have a similar or higher boiling point than the desired bisphenol.
U.S. Pat. No. 4,141,924 discloses a process for purifying a crude crystalline aromatic compound, such as bisphenol A, wherein a dispersion of the liquified crude material in an aqueous liquid is formed by agitating the mixture at ambient pressure and at a temperature sufficient to melt the crude material. Agitation is then reduced to permit formation of three phases: a solid crystalline phase, an aqueous liquid phase and a mother liquor phase. According to one example, bisphenol A of high purity and a relatively high yield is obtained; however, in this example chlorobenzene is used as a solvent. The use of such an organic solvent is not very desirable.
U.S. Pat. No. 4,861,919 describes the purification of aromatic compounds, such as bisphenol A, by a countercurrent multi-stage water crystallization wherein the crystals are melted by the addition of heat and water prior to each crystallization step. The temperature in each crystallizer is selected depending on the desired purity of the compound to be crystallized. The temperature in the first crystallizer is relatively low, preferably about 70.degree. C. Bisphenol A of a high yield, but of relatively low purity, is recovered from the first crystallizer. The obtained crystals are washed and melted at progressively higher temperatures in order to obtain a higher degree of purity. Crystals of extremely high purity may be achieved by this process. However, the process has to be conducted very carefully because the repeated melting of the crystals increases the risk of partial degradation or cracking of the product.
Due to the various disadvantages of the known processes, it is still desirable to provide a new process for the purification of a crude bisphenol.